The present invention generally relates to a driving method and apparatus for driving a reciprocating illumination type imaging system in synchronism with a moving photoconductive member in electrostatic photography.
A variety of instruments are known in which an illumination type imaging system moves along a predetermined path while a member to be imaged by the imaging system moves along a path which is different from the path of movement of the imaging system. In such a type of instrument, it is a prerequisite that the movement of the imaging system and that of the member to be imaged by synchronized or timed accurately to each other. Taking an electrostatic copying machine, for example, problems heretofore pointed out in connection with instruments of the type described will be discussed.
In an electrostatic copying machine, the member to be imaged by the imaging system may comprise a rotatable photoconductive drum as well known in the art. For timed movements of the imaging system and drum, it has been most customary to employ a design wherein power is supplied from a single drive source to both the imaging system and the drum via chains or wires and the imaging system is driven for reciprocation through a reversible clutch positioned in a power transmission line to the imaging system. A problem inherent in this design originates from the use of a single motor as the drive source and chains or like members as the gearing. When the operation speed of the motor is varied significantly or the chain vibrated as a result of a fluctuation of a load, the rotating speed of the drum or the moving speed of the imaging system undergoes a fluctuation which critically deteriorates the resolution of the copier or invites jittering.
An expedient hitherto proposed to settle this problem employs separate drive lines for the drum and imaging systems, respectively. Each of these drive lines is associated with a speed control line which is independent of the other but supplied with reference signals from a common reference oscillator. More specifically, the drum or the imaging system is driven by its own phase locked loop which comprises a phase comparator, an amplifier, a servo motor and an encoder. Though free from the drawback attributable to the use of a single drive source, this expedient requires a disproportionate cost since each of the drum drive line and imaging system drive line must be provided with an exclusive feedback control.
The increase in cost may be avoided by another known expedient which controls the movement of the imaging system based on the movement of the drum. According to this expedient, whereas the drum is driven by a main motor, the imaging system is driven through a phase locked loop as in the first expedient which is electrically connected with an encoder associated with the drum. While such an expedient succeeds in simplifying the construction of the control system to reduce the cost, it still entails a drawback that a fluctuation of a load is directly reflected by that of the main motor, since the main motor drives not only the motor but other various sections of the printer in sequence. With this in view, the movement of the imaging system must be so controlled as to well follow substantial variations in the drum speed. However, the second expedient can not offer such a control but, rather, creates a substantial relative variation in speed between the drum and the imaging system with the resultant degradation to the quality of reproduced images.